This invention relates to the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals which have high solidification temperatures and tend to produce defects caused by uneven solidification at the chilled casting surfaces of the rolls. Much attention has therefore been given to the design of metal delivery nozzles aimed at producing a smooth even flow of metal to and within the casting pool. U.S. Pat. Nos. 5,178,205 and 5,238,050 both disclose arrangements in which the delivery nozzle extends below the surface of the casting pool and incorporates means to reduce the kinetic energy of the molten metal flowing downwardly through the nozzle to a slot outlet at the submerged bottom end of the nozzle. In the arrangement disclosed in U.S. Pat. No. 5,178,205 the kinetic energy is reduced by a flow diffuser having a multiplicity of flow passages and a baffle located above the diffuser. Below the diffuser the molten metal moves slowly and evenly out through the outlet slot into the casting pool with minimum disturbance. In the arrangement disclosed in U.S. Pat. No. 5,238,050 streams of molten metal are allowed to fall so as to impinge on a sloping side wall surface of the nozzle at an acute angle of impingement so that the metal adheres to the side wall surface to form a flowing sheet which is directed into an outlet flow passage. Again the aim is to produce a slowly moving even flow from the bottom of the delivery nozzle so as to produce minimum disruption of the casting pool.
Japanese Patent Publication 5-70537 of Nippon Steel Corporation also discloses a delivery nozzle aimed at producing a slow moving even flow of metal into the casting pool. The nozzle is fitted with a porous baffle/diffuser to remove kinetic energy from the downwardly flowing molten metal which then flows into the casting pool through a series of apertures in the side walls of the nozzle. The apertures are angled in such a way as to direct the in-flowing metal along the casting surfaces of the rolls longitudinally of the nip. More specifically, the apertures on one side of the nozzle direct the in-flowing metal longitudinally of the nip in one direction and the apertures on the other side direct the in-flowing metal in the other longitudinal direction with the intention of creating a smooth even flow along the casting surfaces with minimum disturbance of the pool surface.
After an extensive testing program we have determined that a major cause of defects is premature solidification of molten metal in the regions where the pool surface meets the casting surfaces of the rolls, generally known as the "meniscus" or "meniscus regions" of the pool. The molten metal in each of these regions flows towards the adjacent casting surface and if solidification occurs before the metal has made uniform contact with the roll surface it tends to produce irregular initial heat transfer between the roll and the shell with the resultant formation of surface defects, such as depressions, ripple marks, cold shuts or cracks.
Previous attempts to produce a very even flow of molten metal into the pool have to some extent exacerbated the problem of premature solidification by directing the incoming metal away from the regions at which the metal first solidifies to form the shell surfaces which eventually become the outer surfaces of the resulting strip. Accordingly, the temperature of the metal in the surface region of the casting pool between the rolls is significantly lower than that of the incoming metal. If the temperature of the molten metal at the pool surface in the region of the meniscus becomes too low then cracks and "meniscus marks" (marks on the strip caused by the meniscus freezing while the pool level is uneven) are very likely to occur. One way of dealing with this problem has been to employ a high level of superheat in the incoming metal so that it can cool within the casting pool without reaching solidification temperatures before it reaches the casting surfaces of the rolls. In recent times, however, it has been recognised that the problem can be addressed more efficiently by taking steps to ensure that the incoming molten metal is delivered relatively quickly by the nozzle directly into the meniscus regions of the casting pool. This minimises the tendency for premature freezing of the metal before it contacts the casting roll surfaces. It has been found that this is a far more effective way to avoid surface defects than to provide absolutely steady flow in the pool and that a certain degree of fluctuation in the pool surface can be tolerated since the metal does not solidify until it contacts the roll surface. An example of this approach is to be seen in Japanese Patent Publication No 64-5650 of Nippon Steel Corporation.
Although the direction of molten metal from the delivery nozzle directly to the meniscus regions of the casting pool allows casting with molten metal supplied with relatively low level of superheat without the formation of surface cracks, problems can arise due to the formation of pieces of solid metal known as "skulls" in the vicinity of the pool confining side plates or dams. These problems are exacerbated as the superheat of the incoming molten metal is reduced. The rate of heat loss from the melt pool is greatest near the side dams due primarily to additional conductive heat transfer through the side dams to the roll ends. This high rate of local heat loss is reflected in the tendency to form "skulls" of solid metal in this region which can grow to a considerable size and fall between the rolls causing defects in the strip. Because the net rate of heat loss is higher near the side dams the rate of heat input to these regions must be increased if skulls are to be prevented. There have been previous proposals to provide an increased flow of metal to these "triple point" regions by forming galleries in the upper part of a delivery nozzle which receive a separate flow of metal from the tundish as seen for example in the delivery nozzle disclosed in our granted U.S. Pat. No. 5,221,511. However, this requires the formation of complex gallery passages and a high level of superheat of the molten metal because of cooling of the metal in the galleries. By the present invention the necessary increase in net heat input can be achieved by the simple expedite of increasing the supply of molten metal to the end regions of the roll by flow of molten metal through outlets formed in the two ends of the nozzle.